REAGENTS. 813 



examining the object always hold the sHde with the left hand, 

 and use the right hand for maintaining the proper focus by means 

 of either the coarse or fine adjustment. 



8. In all cases where practicable make drawings of the sec- 

 tions examined. 



9. In some cases it is desirable to apply a reagent after the 

 material has been mounted, as in the addition of an iodine solution 

 to a section to determine the presence of starch, and this is 

 accomplished by placing a drop or two of the reagent, by means 

 of a pipette or dropper, near the edge of the cover on one side 

 and taking up tli^e excess of liquid by temporarily placing a piece 

 of filter paper on the opposite side (Fig. 324). 



Air-Bubbles. The beginner in the use of the microscope is 

 often confused by the presence of air-bubbles, mistaking them 

 'for portions of the material under examination, as starch grains, 

 oil-globules or even the cells themselves. While it is not prac- 

 ticable to avoid their presence entirely, their identity may be 

 determined by the manner of focussing upon them. When 

 focussing above on an air-bubble it always appears dark (Fig. 

 325, C), but when the focus is lowered, it becomes lighter 

 (Fig. 325, D) ; while in the case of an oil-globule or starch grain 

 the reverse is true, i.e., it is lightest when the focus is above 

 (Fig. 325, E) and darker when the focus is lowered (Fig. 325, 

 F). To obviate as much as possible the formation of air-bubbles, 

 the edge of the cover-glass should first be applied to the liquid 

 on one side and then allowed to drop upon it. When particular 

 care is required, a pair of forceps may be used for holding the 

 cover and lowering it gradually. 



Frequently also simple pores in the cell-walls are mistaken 

 for cell-contents, and sometimes even the lumen of the cell has 

 been mistaken for a prism of calcium oxalate. The beginner 

 will therefore find it an advantage to study the simple pores in 

 the pith cells of elder or sassafras (Fig. 326). In sections show- 

 ing either the upper or lower wall of the cells, the pores appear 

 as circular or elliptical markings, which may be mistaken for cell- 

 contents, but which in focussing upon them are seen to be optical 

 or microscopical sections of the pores. 



Micrometry or Microscopic Measurement. In the micro- 



